1. Field of the Invention
This invention relates to a gear grinding machine arranged to perform gear grinding (gear machining) using a threaded grinding wheel, and equipped with a dressing device for dressing the threaded grinding wheel. The gear grinding machine can easily correct an error (dimensional error), if any, in a ground area.
2. Description of the Related Art
A gear grinding machine, in which a gear-shaped workpiece after heat treatment is ground by “a threaded grinding wheel”, a gear grinding tool, to finish a gear, has so far been known. The threaded grinding wheel is an annular grinding wheel having threads (rack teeth) formed spirally on its outer peripheral surface. Grinding is performed by numerically controlling the positions in an orthogonal coordinate system (positions on the X-axis, Y-axis and Z-axis) of the threaded grinding wheel, the rotational speed of the threaded grinding wheel, and the rotational speed of a table on which the work (gear) has been installed.
As grinding proceeds, the threaded grinding wheel wears, and its sharpness decreases. Thus, after the threaded grinding wheel has ground many gears continuously, the worn-out threaded grinding wheel needs to be dressed by a dressing device to regenerate a sharp cutting edge.
Some gear grinding machines are equipped with dressing devices. Among the dressing devices is a rotary dressing device provided with a rotationally driven disk-shaped dressing tool. With this rotary dressing device, the disk-shaped dressing tool is kept rotationally driven, and brought into contact with the flank of the thread of the threaded grinding wheel being rotated, thereby carrying out dressing.
If the shape of the ground gear is not the target shape, but has a shape error, actions of the gear grinding machine need to be modified.
In this case, the “gear shape error” includes a “tooth profile error” and a “helix form deviation”.
To correct the “tooth profile error” of the gear to be ground, namely, to modify its tooth profile (tooth profile pressure angle), the wheel pressure angle of the threaded grinding wheel has to be modified. Modification of the wheel pressure angle of the threaded grinding wheel is made by dressing the threaded grinding wheel by the dressing device.
To correct the “helix form deviation” of the gear to be ground, on the other hand, it is common practice to modify synchronous motions made when the gear (work) placed on a rotating table is ground by the threaded grinding wheel. That is, the helix form deviation can be corrected by modifying synchronization between motions of the threaded grinding wheel moving in a Z-axis direction (i.e., vertical direction), and rotary motions of the table, on which the work is placed, during grinding.
Concretely, the moving motion in the Z-axis direction of the threaded grinding wheel is slowed, or the rotary motion of the table is quickened, whereby the helix angle of the gear is increased. On the other hand, the moving motion in the Z-axis direction of the threaded grinding wheel is increased, or the rotary motion of the table is slowed, whereby the helix angle of the gear is decreased. Thus, the synchronization between the moving motion in the Z-axis direction of the threaded grinding wheel and the rotary motion of the table is adjusted, whereby the helix angle of the gear can be arbitrarily set and modified. By so adjusting the synchronization, the helix angle of the gear to be ground can be modified to correct the helical form deviation.
One of methods for modifying the wheel pressure angle is to turn (turn about a vertical axis (Z-axis)) the disk-shaped dressing tool in contact with the thread of the threaded grinding wheel, although details will be described later.
Among gear grinding machines, therefore, are those of the type having a mechanism for turning the rotary dressing device. With the gear grinding machine having such a turning mechanism, an operator manually turns the rotary dressing device (dressing tool) about the Z-axis by use of a block gauge, which is a tool for turning, to modify the wheel pressure angle.
When the wheel pressure angle of the threaded grinding wheel is modified to correct the tooth profile error, the tooth profile can be modified. Such correction of the tooth profile error does not affect the form of the tooth trace.
If the synchronization between the moving motion in the Z-axis direction of the threaded grinding wheel and the rotary motion of the table is modified to correct the helix form deviation, on the other hand, the form of the tooth trace can be modified. Such correction of the helix form deviation inevitably results in the modification of the tooth profile. Since the correction of the helix form deviation leads to modification of the tooth profile, as described above, an unintended tooth profile may be obtained.
The present invention has been accomplished in light of the above-described problems with the earlier technologies. It is an object of the present invention to provide a gear grinding machine which can appropriately correct a tooth profile error and a helix form deviation in anticipation of a change in a tooth profile according to correction of the helix form deviation, if there is an error or deviation of the tooth profile and the tooth trace form.